If you can't seem to get the air out, I would suspect three areas - pump section gaskets not sealing well, bad or missing hydraulic manifold sealing o-rings, manifold mating surfaces not tight enough. Tip - only use the New Holland o-rings here - not the aftermarket or hardware store versions. It DOES make a difference.
I had to bleed my 1954 NAA piston pump quite a bit after the rebuild. But since bleeding it initially, I have not had another problem. Open the plug until just a few threads are engaged, run the engine at the lowest possible RPM until nothing but oil squeezes out between the threads of the plug. Get a big pan underneath there - it is messy and will take quite a bit to get all the air out.